How to relocate a library using Python in 2.80 beta?
Going step by step in the native Blender code using a debugger, I was able to figure out the properties to give to the operator (which is wm.lib_relocate):

bpy.ops.wm.lib_relocate(
    library="cube-v02.blend",
    directory="C:\\Users\\someone\\Desktop\\",
    filename="cube-v01.blend"
)


Where:


library is the name of the library to relocate which seems to be the file name (without the directory part).
directory is the directory where the new library (blend file) is located.
filename is the file name of the blend file.